Incremental optical shaft angle encoders are used for resolving the position or measuring rotation of a shaft. Exemplary shaft angle encoders are known from U.S. Pat. Nos. 4,451,731 and 4,691,101, for example. Such encoders include a light source for emitting a light beam, an encoder wheel for modulating the light beam in response to shaft rotation, and a photodetector assembly for receiving the modulated light and producing electrical signals indicating the amount of light received by the photodetectors.
As the light is modulated in response to shaft rotation, each electrical signal from the photodetector assembly produces a waveform The position of the shaft determines the position of each signal on its particular waveform, i.e., the phase of each signal. Therefore, the electrical signals from the detectors can be used to indicate shaft rotation Further, two or more properly out of phase signals, from separate photodetectors, can be used to indicate both direction and magnitude of rotation.
Some shaft angle encoders have a code wheel with alternating opaque and transparent areas, and the light source and photodetectors are on opposite faces of the code wheel. Other shaft angle encoders have reflective areas so that the light source and photodetectors can be on the same face of the code wheel.
One such encoder, disclosed in U.S. Pat. No. 4,952,799 issued to Loewen, provides a shaft angle encoder having a rotatable code wheel with a plurality of alternating reflective and nonreflective areas extending in a circumferential path around the wheel. A light emitter is provided for illuminating the reflective areas of the wheel and a plurality of photodetectors are arrayed on the same side of the code wheel as the light emitter for receiving modulated light which is directly reflected from the reflective areas on the code wheel. This produces a plurality of electrical signals indicative of code wheel position.
While the above-described encoder provides several advantages over the prior art, including relatively inexpensive cost and relatively small size, such an encoder having a light emitter and photodetectors on the same substrate suffers from certain drawbacks. In some cases light from the light emitter reflects off of the front surface of the encapsulation of the light emitter and onto the adjacent photodetectors. This causes an undesirable signal to occur on the photodetectors which lowers the overall accuracy of the encoder. Moreover, a relatively high light emitter drive current must be used to overcome these internal reflections and the operating distance between the sensor and reflective code wheel must be kept to a minimum. It is desirable to eliminate these undesirable effects while preserving the advantages of having the light emitter and photodetectors arrayed on the same substrate.